It is well known that a nitrile group-containing highly saturated copolymer rubber which may be obtained by hydrogenating the carbon-carbon double bonds of an acrylonitrile-butadiene copolymer rubber has good heat resistance, weather resistance and oil resistance. Accordingly, there have been attempts to exploit such desirable properties of the copolymer rubber in the preparation of thermoplastic elastomer compositions by blending said rubber with a synthetic resin such as a polyamide (see Japanese Unexamined Patent Publication Nos. 269138/1990, 191656/1990 and 112656/1988). These compositions possess a good balance of such desirable properties as high impact resistance, resistance against aging by heat, weather resistance and mechanical strength.
However, in case the viscosity of a nitrile group-containing highly saturated copolymer rubber is greatly different from that of a synthetic resin to be blended therewith, a mixture composition results due to the incompatibility between them. Therefore, it is required to improve, in particular, the processibility of the nitrile group-containing highly saturated copolymer rubber.
It is suggested a method for imparting good processibility to a nitrile group-containing highly saturated copolymer rubber through lowering the Mooney viscosity of the rubber by a high shear treatment (Japanese Unexamined Patent Publication No. 122103/1991). This suggested method comprises the step of subjecting a hydrogenated nitrile rubber having a Mooney viscosity of 55 to 100 to high shear conditions in the presence of an oxygen donor, e.g., oxygen or a peroxide, as well as a radical transfer agent. The shearing induces thermal degradation of the rubber, thereby lowering the Mooney viscosity of the rubber to the range of 30 to 50.
In this method, the oxygen donor functions to stabilize free radicals generated during the thermal degradation. However, the Mooney viscosity of the rubber produced by this method is known to increase during storage, even in the process of an added anti-oxidant, due to gelation caused by residual peroxides as well as by carboxyl groups, carbonyl groups and others generated by the high shear treatment.
Accordingly, it is an object of the invention to improve the physical properties of a mixture composition obtained by blending a thermoplastic or thermosetting resin with a nitrile group-containing highly saturated copolymer rubber, by way of enhancing the compatibility of said rubber with the resin.
The present inventors have endeavored to develop polymeric compositions that meet the afore-mentioned objective and found that the impact resistance of a mixture composition of a polyamide resin and a nitrile group-containing highly saturated copolymer rubber can be improved over that of a conventional blend, by way of pretreating the hydrogenated acrylonitrile-butadiene copolymer rubber under high shear conditions in a twin screw extruder in the presence of an anti-oxidant to lower its Mooney viscosity to a specific level.